In recent years, with increasing transmission traffic, demand for larger capacities of optical transmission systems is increasingly on the rise. In short-distance transmission systems, not only larger capacities but also low-cost and simple configurations are demanded.
Thus, an application of a discrete multi-tone (DMT) modulation scheme to an optical transmission system has been discussed. The DMT modulation scheme is one of multi-carrier transmission technologies based on an orthogonal frequency division multiplexing (OFDM) technology and is used in a digital subscriber line (DSL) technology such as an asymmetric DSL (ADSL).
The DMT modulation scheme uses a “bit loading” which allocates bit numbers to each subcarrier in accordance with its transmission characteristic. Examples of the transmission characteristic may include a signal to noise ratio (SNR) and/or a bit error rate (BER). The transmission characteristic may also be referred to as a transmission condition. For example, more bits are allocated to a subcarrier with a high transmission characteristic than to a subcarrier with a low transmission characteristic. Thus, it is possible to improve a spectral efficiency.
When the DMT modulation scheme is installed in an optical transmission system, an electric/optic (E/O) converter using a direct modulation scheme for a semiconductor laser is applicable to an optical transmitter. Further, an optic/electric (O/E) converter using a light reception element such as a PD (photo detector or photo diode) is applicable to an optical receiver.
The semiconductor laser and the PD are general-purpose optical devices and thus are inexpensive. In the direct modulation scheme, a modulated signal is generated by modulating a drive current of the semiconductor laser that is a light source with transmission information. When compared with an external modulation scheme using an optical modulator in addition to a light source, the direct modulation scheme is difficult in a speedup but is possible to achieve downsizing and cost reduction because it is unnecessary to use phase information of a signal light. The semiconductor laser used in the direct modulation scheme may be referred to as a direct modulation laser (DML).
Therefore, by installing the DMT modulation scheme in an optical transmission system, it is possible to provide an optical transmission system available to improve a spectral efficiency (in other words, improve transmission capacities) and available to achieve downsizing and cost reduction.
As examples of optical transmission technology, there are technologies described in JP 11-127119 A and JP 6-303196 A.
For example, in a transmitting side, a transmission analog signal is multiplexed with a pilot signal which has a single frequency in a frequency band far apart from that of the transmission analog signal and has a fixed amplitude and the multiplexed signals are transmitted to an optical transmission line. Then, variations of a transmission gain in the optical transmission line are monitored by monitoring the pilot signal halfway through the optical transmission line and a gain of a variable gain amplifier provided on the optical transmission line is controlled so that the monitored value is constant.
As another example, an optical reception unit detects an instantaneous worst value of distortion of a reception signal to feedback the detected value to an optical transmission unit. The optical transmission unit determines a modulation level of a frequency multiplexed digital signal light to be transmitted and the number of frequency division multiplexing channels based on the received instantaneous worst value.
In an optical transmission system using the direct modulation scheme, a transmission characteristic changes depending on drive conditions (for example, the amplitude and a bias current of a drive current) of a semiconductor laser that is an example of a light source. Thus, it is preferable to optimize the drive conditions of the semiconductor laser to ensure transmission characteristics expected for an optical transmission system.
Further, unlike an NRZ (Non-Return-to-Zero) scheme, since the DMT modulation scheme changes a bit allocation (or multi-level modulation format) and/or transmission power of each subcarrier in accordance with the transmission characteristics, it is preferable to optimize the drive conditions of the semiconductor laser for each transmission characteristic.
In the technologies described above, however, no discussion has been made in an application of the DMT modulation scheme to an optical transmission system. Therefore, no discussion has also been made in optimizing drive conditions of the semiconductor laser in accordance with the transmission characteristics.